A Study to Learn About the Effects of Sisunatovir in Infants With Respiratory Syncytial Virus Lower Respiratory Tract Infection.

The purpose of this clinical trial is to learn about the safety and effects of the study medicine (sisunatovir). Sisunatovir is developed as potential treatment of Respiratory Syncytial Virus (RSV) infections. This study will assess sisunatovir as compared to placebo in infants aged 1 month to 36 months who are hospitalized with RSV lower respiratory tract infection (LRTI). A placebo looks like the study medicine but does not contain any active medicine in it. This study will be conducted in 3 parts: In Part A participants aged 6 months to 3 years will be given a single dose of 2.5 mg/kg of sisunatovir in Cohort 1. In Cohort 2, participants age 1 month to 6 months will receive a single dose of 2 mg/kg of sisunatovir only after the completion of Cohort 1. 12-24 participants will be enrolled in Part A In Part B participants age 1 month to 36 months will receive sisunatovir or placebo dosed every 12 hours for 5 days. Doses for part B will be determined after the completion of Part A. 24-40 participants will be enrolled in Part B. The dose regimen for Part C will be determined after the completion of Part B. Approximately 120 participants age 1 month to 36 months will receive either sisunatovir or placebo. To participate in this study participants must meet the following criteria: 1. Age 1 month to 36 months 2. Weight ≥ 3.5 kg 3. Diagnosis of LRTI 4. Diagnosis of RSV 5. Hospitalization due to RSV LRTI